1. Field of the Invention
The present invention relates to measurements of thin films deposited during a chemical vapor deposition. In particular, the invention relates to measuring a gaseous environment during deposition. The invention is used as a quality control monitor during the deposition process.
2. Description of the Related Art
The integrated circuit industry uses thin films of titanium nitride (TiN) for many applications. One application of TiN films is as a diffusion barrier, and glue layer for submicron contacts and vias. It prevents the inter-diffusion of silicon and aluminum in an aluminum plug process. TiN also serves as a glue layer for the tungsten plug process and it prevents WF.sub.6 from attacking the underlying metal or silicon during tungsten deposition. As contact and via windows shrink, physical vapor deposition techniques for depositing TiN are being replaced by chemical vapor deposited (CVD) TiN to meet step coverage requirements for contacts and vias below 0.35 .mu.m.
The CVD deposited TiN films provide better step coverage than sputtered TiN films for via and contacts opening below 0.5 microns. The CVD TiN process studied in this application resulted from the thermal deposition of tetrakis (dimethylamido) titanium (TDMAT), optionally followed by post deposition plasma treatment.
Deposition rate monitoring techniques often are performed by reiterative techniques and require test wafers. In typical film deposition processes, one or more test wafers are measured to determine if the process is within normal parameters. In most cases the parameters of interest are measured after the film has been deposited on the substrate. If the measured parameters are not within the desired tolerances, the process parameters are adjusted and more test wafers are measured to assure process compliance. The qualification procedure is generally repeated periodically to assure stable performance, in some cases on a weekly or daily basis. Between the qualification tests, the process tool must be assumed to be stable.
However, the reiterative technique described above has a number of disadvantages. It is time consuming and inefficient compared to real time deposition rate monitoring techniques. Reiterative techniques require test wafers to be created instead of product wafers. Test wafers cannot be sold and therefore displace the revenue stream generated by the production of product wafers.
In addition to the down time of the deposition chamber while the test wafers are being made, the deposition chamber must remain idle while conformance measurements are made on the completed test wafers.
Furthermore, test wafers allow the detection of errors only once the test wafers have been made and measured. If the test wafer process is performed only once every twenty-four hours, and the deposition chamber defect has occurred twenty-three hours ago, then approximately 96% of the production wafers made that day are defective.
These problems suggest that real time, in situ process monitoring should be used instead of the test wafer reiterative monitoring process. With optical in situ monitoring, either the detector itself must be within the deposition chamber or it must have a window. However, when the deposition is performed, deposited films also coat other things within the chamber, including the detector or the inside surface of the window. This obscures the optical measurements. Thus, there is a need for non-optical in situ process monitoring.
Furthermore, there is a need to monitor the input gases of the deposition process to ensure that the mass flow controller is working properly.